As the old standard TV technology (CRT) has nearly reached its limits, some new display technologies (LCD, PDP, . . . ) are encountering a growing interest from manufacturers. Indeed, these technologies now make it possible to achieve true flat displays with very limited depth.
Referring to the last generation of European TV, a lot of work has been made to improve its picture quality. Consequently, the new technologies have to provide a picture quality as good or better than standard TV technology. On the one hand, these new technologies allow to produce flat screens, of really attractive thickness, but on the other hand, they generate new kinds of artefacts, which could reduce the picture quality. Most of these artefacts are different as for CRT-TV pictures and so more visible since people are used to seeing the old TV artefacts unconsciously.
One of these new artefacts are “sticking” or “ghost” images due to the “marking” or “burn-in” effect. This effect is generated by a change of the panel behaviour depending on the time a specific picture has been displayed. In other words, if a picture has been displayed a long time on a screen, the picture will stay visible as a shadow on every other scene: this is called “ghost” image. As an example, if a black picture with white text is displayed on a screen during a significant time, the text will be readable on another scene later. This affects different display technologies like CRT and PDP in different ways. In the case of the plasma display technology, this effect is quite strong and leads to very disturbing artefacts. These effects are very critical for all professional applications (notice board . . . ) displaying a lot of static pictures as well as consumer applications (PC images, digital photo, etc.).
A PDP utilizes a matrix array of discharge cells which can only be “ON” or “OFF”. Also unlike a CRT or LCD in which gray levels are expressed by analog control of the light emission, a PDP controls the gray level by modulating the number of light pulses per frame (sustain pulses). The eye will integrate this time-modulation over a period corresponding to the eye time response. To perform a grayscale rendition, the Plasma display is commonly divided in sub-lighting periods called sub-fields each one corresponding to a bit of the input video picture data. For instance, if 8 bit luminance levels are provided, in that case each level will be represented by a combination of the 8 following bits :1-2-4-8-16-32-64-128.
To realize such a coding with the PDP technology, the frame period will be divided in 8 lighting periods (called sub-fields), each one corresponding to a bit. The number of light pulses for the bit “2” is the double as for the bit “1”, and so forth. With these 8 sub-periods, we are able through sub-field combination, to build the 256 gray levels. A simple sub-field organisation is shown in FIG. 1.
For clarification, a definition of the term sub-field is given here: A sub-field is a period of time in which successively the following is being done with a cell:                1. There is a writing/addressing period in which the cell is either brought to an excited state with a high voltage or left in its neutral state with lower voltage.        2. There is a sustain period in which a gas discharge is made with short voltage pulses which lead to corresponding short lighting pulses. Of course only the cells previously excited will produce lighting pulses. There will not be a gas discharge in the cells in neutral state.        3. There is an erasing period in which the charge of the cells is quenched.        
In some specific plasma driving schemes (incremental coding, e.g. the CLEAR concept announced by Pioneer) the addressing or erasing periods are not present in each sub-field. Instead, a selective addressing/erasing is performed ahead or after a group of sub-fields.
Often, more sub-fields are used to reduce moving artifacts and “priming” could be used on more sub-fields to increase the response fidelity. Priming is a separate optional period, where the cells are charged and erased. This charge can lead to a small discharge, i.e. can create background light, which is in principle unwanted. After the priming period an erase period follows for immediately quenching the charge. This is required for the following sub-field periods, where the cells need to be addressed again. So priming is a period, which facilitates the following addressing periods, i.e. it improves the efficiency of the writing stage by regularly exciting all cells simultaneously.
On a PDP, two kinds of ghost images are existing:                “Short term burning”: this ghost image (3 to 5% of its original brightness) is mainly a positive image (“burned” cells are brighter than others) which will disappear after a short time (some minutes up to some hours). The origin is not completely clear yet but it seems that this effect is related to some kind of charges which have been accumulated during the time a cell stays ON. Later these charges improve the luminance emitted by the cell even if only priming is active in the frame period. An example of this effect is shown in FIG. 2, where a static menu is displayed during some minutes and afterwards a “ghost” menu is visible on the next scene as a positive image.        “Long term burning”: this stable sticking image is a negative image (“burned” cells are darker than others) related to a kind of aging of the plasma cell. The cumulative amplitude can go up to 50% loss of luminance. This effect is illustrated in FIG. 3, where the static menu is displayed during some hours and then a “ghost” menu is visible on the next scene as a negative image.        
The long term burning is the more critical issue since this effect is not reversible and could reach 50% luminance loss. This effect is linked to a kind of aging of the PDP represented in FIG. 4. At the beginning of the PDP lifetime, the aging process is quite strong and leads quickly to disturbing ghost images above all for professional applications using static pictures. Later this process will decrease.
In the case of the CRT technology, this effect is really an issue for PC monitors which dispose today of a screen saver in order to prevent a strong marking of the screen.
Numerous PDP manufacturers are trying to modify the technology in order to avoid this problem (gas mixing, MgO layer thickness, cell geometry) but up to now, no real improvement has been announced.
One approach is to use a kind of jittering in picture position on professional PDPs. Thereby the picture is regularly translated a bit in all directions. This will lead to less sharpness of the ghost image but it still exists.
Another approach is to invert the static pictures in order to burn the entire PDP panel in the same way. This requires to know the picture content and this method is strongly limited by the power limitation of the panel.